This invention relates to bioprosthetic valves and more particularly to a method of fabricating expansible bioprosthetic valve stents.
Two basic types of artificial heart valves are available for replacement of diseased human heart valves. The first type, mechanical valves, are constructed of synthetic rigid materials such as plastic or metal. Their use is associated with thrombogenesis, requiring valve recipients to be on long term anti-coagulation.
The second type, tissue valves or bioprosthetic valves, consist of valve leaflets of preserved animal tissue mounted on an artificial support or xe2x80x9cstentxe2x80x9d.
The durability of bioprosthetic heart valves is limited to about 12 to 15 years. The limitations in the long term performance of bioprosthetic heart valves are believed to be due largely to the mechanical properties of the valve and the stresses imposed on the tissue leaflets by the rigidity of the stent structure while the aortic root to which the artificial valve is attached expands and contracts during the cardiac cycle. An important feature of the natural heart valve is its ability to expand in diameter by more than 10% during systole. This ability of the aortic root to expand facilitates blood flow due to a better opening of the valve during systole and contributes to minimal bending of the cusps, thus reducing possible internal flexural fatigue.
In an attempt to overcome the rigidity of artificial heart valves and accommodate the expansion of the aortic root during systole, a bioprosthetic heart valve with pivoting stent posts has been devised (Canadian Patent No. 2,123,824).
U.S. Pat. No. 5,258,023 discloses a prosthetic heart valve in which the valve leaflets are fashioned from synthetic materials, in an attempt to avoid the mechanical failure of natural tissue leaflet material.
Although these designs allowed for improved hemodynamics, they did not totally solve the problems arising from the rigidity of artificial heart valve stents. There remained a need for an artificial heart valve stent that is expansible, resilient and tough and which provides a better opening of the valve during systole to facilitate blood flow and contributes to minimal bending of the cusps to reduce valve failure.
The present invention provides a bioprosthetic stent fabricated from a hydrogel. A hydrogel stent combines sufficient strength for use in a bioprosthetic valve with pliability and elasticity characteristics which much more closely resemble those of the aortic root than previously available valve stents.
The invention provides a bioprosthetic valve stent fabricated from a hydrogel, wherein the hydrogel, when hydrated, has
(a) a strain value at ultimate tensile stress (UTS) greater than the maximum strain occurring in a human aortic root under physiological conditions;
(b) an elastic modulus similar to that of the aortic root; and
(c) a relaxation rate similar to that of the aortic root.
The invention provides a method of fabricating a bioprosthetic valve stent comprising the steps of
(a) preparing a mold cavity corresponding in shape to the stent to be fabricated;
(b) filling said mold cavity with a solution of a selected hydrogel;
(c) allowing the hydrogel to solidify; and
(d) removing the solid hydrogel from the mould cavity and hydrating the solidified hydrogel for a suitable period of time.
The invention provides a bioprosthetic heart valve comprising
(a) a stent in accordance with any of claims 1 to 5,
(b) leaflet valve means having three generally triangular leaflets defining respective cusps which are adapted to open and close during heart systole and diastole respectively; and
(c) means for attaching the leaflet valve means to the stent.